Production of large-sized GaN crystals is still difficult. Wide GaN substrate wafers having diameters larger than 50 mm cannot be produced at low cost with mass production. Surface planar-processing (grinding, lapping and polishing) of GaN wafers is also difficult. There are no prior art with regard to the surface planar-processing of GaN wafers. No prior references related to the surface planar-processing of GaN wafers can be cited yet.    (1) Japanese Patent Laying Open No. 2004-165360 proposed a wafer/plate-adhesion method of sprinkling a liquid wax onto a polishing plate by a spray, pushing GaAs wafers onto the plate and fixing the GaAs wafers on the plate. It was a proposal of a preliminary wafer/plate gluing method for polishing one surface of GaAs wafers. There are plenty of improvements in polishing techniques of Si substrate wafers and GaAs substrate wafers. However, there are few proposals for polishing techniques of group 3 nitride substrate wafers.    (2) Japanese Patent Laying Open No. 2002-222785 proposed a GaAs wafer/plate gluing method of sticking OF(Orientation Flat)-carrying GaAs wafers, whose OFs are cleavage planes, in directions of the OFs facing inward. The purpose of (2) is to protect weak cleavage plane OFs from excess wear-out by facing the cleavage plane OFs inward. Since rotation speeds are the least at the innermost spot of a wafer, polishing force is the weakest at the innermost of a wafer. If OF is placed at the innermost spot, the weak OF would be protected from the polishing force. Prior references (1) and (2) proposed improvements of pasting GaAs wafers on a plate. This invention relates not to GaAs but to the group 3 nitride substrate wafers. OF is not necessarily a cleavage plane in the present invention.
Circular wafers are allotted with orientation flats (OFs) or identification flats (IFs) for designating crystallographic orientation and obverse/reverse distinction.
(2) Japanese Patent Laying Open No. 2002-222785 proposed a GaAs wafer having an orientation flat (OF) that is worn out by 1.5 μm to 10 μm by polishing. When surface roughness of an obverse surface is different from that of a rear surface, the obverse/reverse surfaces can be discerned by the difference of surface roughness with human eyesight. The single orientation flat (OF) is enough to indicate crystal orientation in the case. When both surfaces have similar surface roughness and the obverse/reverse is not discriminated with eyesight, another flat is further required in addition to the orientation flat (OF) for discriminating the top/bottom and the orientation. Another flat is named here an identification flat (IF).